1. Field of the Invention
This invention relates to carbon current collecting shoes with damage detector, sometimes also referred to by other names such as "wearing strip" or "pantograph pad". These current collecting shoes are used to transfer electrical energy between a fixed electrical power distribution means, typically a catenary cable, and a mobile device consuming electrical energy, typically an electric train locomotive, which is fitted with a pantograph carrying said carbon current collecting shoe through a sheath fixed to said pantograph.
2. Description of Related Art
A number of patents have already been submitted describing carbon current collecting shoes comprising a damage detection device. The current collecting shoe comprises a sheath and a wearing strip usually provided on its inside surface with a slot containing or forming a sealed longitudinal chamber containing a fluid, such that if excessive wear occurs in said wearing strip, or if it breaks, then said chamber will no longer be sealed causing a leakage or a change of pressure in said fluid, the leak or pressure change in said chamber being detected, causing withdrawing of the pantograph and said current collecting shoe before any excessive damage is caused to the energy transfer equipment and particularly before the catenary wire or cable is damaged.
In British patent GB 1 374 972, said chamber is a tube made of silicon rubber or any other material, and particularly plastics such as PTFE, capable of resisting an operating temperature of, for example, 200.degree. C.
In French patent FR-A1-2 663 592 in the applicant's name, said chamber is a tube made of carbonaceous material with the same mechanical properties as said wearing strip, in order to improve the detection sensitivity, particularly to the appearance of cracks in said wearing strip.
In U.S. Pat. No. 5,189,903 (=EP 415 972), said chamber is formed by the walls of said slot that are sealed by means of a sealing layer and by galvanic deposition of copper on said wearing strip and said slot.
In European patent EP-A1-402 666, said chamber is formed by the upper wall of said slot sealed using a resin, and by an Al or Cu or alloy metallic section open at the top.
The applicant has observed two types of problems when state-of-the-art current collecting shoes are used, and particularly current collecting shoes according to French patent FR-A1-2 663 592. The first problem is excessive sensitivity to the development of cracks in said wearing strip, and the second is its unsuitability for distributing electrical power at low voltage (and high current).
Firstly, it was found that unjustified detection occurs when there is a crack in said wearing strip when said wearing strip is still in working conditions; this is a prohibitive defect since it is unacceptable for a train to stop in the open country due to a simple crack in a wearing strip, since many cracks do not automatically and quickly develop towards damage or a risk of damage that would justify changing the current collecting shoe, the damage typically consisting in the rupture or breaking of the wearing strip and separation of blocks of carbonaceous material forming said wearing strip.
Secondly, particularly with Trans-European high speed trains it has been found that state-of-the-art current collecting shoes are not suitable for all types of electrical energy distribution systems in all the countries passed through, and particularly they are not suitable for an electrical power supply at relatively low voltage, for example 1.5 kV (but with a high current, for example 1100 A or more). In the latter case, and particularly with current collecting shoes made according to French patent FR-A1-2 663 592, a very high increase in the current collecting shoe temperature has been observed, which may reach or exceed 500.degree. C. which would appear to be prejudiciable to the life of the current collecting shoes and the strength of the bond between the wearing strip and the sheath (weld, bonding, etc.) and consequently reducing the electrical current between the wearing strip and the sheath, the role of the sheath being to support said wearing strip and usually also to conduct the electrical current between the wearing strip and the input to the motor power supply circuit in the locomotive.
Therefore, the object of this invention is a carbon current collecting shoe with a damage or wear detector which is not sensitive to simple cracks and which is suitable for the various electrical power supplies, and particularly a low voltage and therefore high current power supply particularly in the case of Trans-European high speed trains (TGV).
Furthermore, the current collecting shoe according to the invention must satisfy other criteria or constraints imposed by their use, in particular weight and aerodynamics, so that the current collecting shoe has a stable behavior at all speeds, and particularly at very high speed.